Various approaches may be used for reducing soot in engine exhausts, including soot filters. One type of soot filter is a diesel particulate filter (DPF). A DPF is a device designed to remove diesel particulate matter from the exhaust gas of a diesel engine. However, collection of particulate matter on the DPF increases back pressure in the exhaust system, decreasing fuel economy. Therefore, the filter may be periodically cleaned or regenerated so that it may continue to be used. Regeneration burns off the accumulated particulate matter either passively, in DPFs that are constructed with a catalyst, or actively, by increasing exhaust temperature to further combust the soot. Exhaust temperature may be increased through engine management, a fuel burner, a catalytic oxidizer, resistive heating coils, or microwave energy, to name a few.
Increases in exhaust temperature, such as during DPF regeneration, may result in self-sustained and self-induced temperature increase, thus increasing the temperature of the emissions control system past acceptable levels and thereby degrading aftertreatment catalysts including diesel particulate filters, lean NOx traps, in-situ Lean NOx Trap and SCR catalysts, decreasing the lifespan and effectiveness of such filters and catalysts. In particular, once DPF regeneration is commenced, the heat of that regeneration can further ignite additional regeneration, thereby leading to self-sustained and rapidly increasing regeneration rates and significantly increased temperatures.
Various approaches may be used to limit DPF temperature, and there inventors herein have recognized issues therewith. For example, exhaust air flow may carry heat away from the exhaust system, but may not provide sufficient cooling to reduce self-sustained and self-induced temperature increases. Additionally, it may not be clear in advance of self-sustained and self-induced temperature increases if certain operating conditions will allow enough air flow to cool the DPF. Another way to control temperature is to limit excess oxygen in the exhaust system, thereby decreasing combustion and the temperature in the system. However, decreasing combustion may stall or stop regeneration prematurely, thereby decreasing the effectiveness of the regeneration, defeating the purpose of the filter and increasing fuel consumption inefficiencies. Further, the reduction of oxygen necessary to limit temperature may lead to excessive smoking in the engine.
The inventors herein have also realized that these concerns and others may be addressed by a system for an engine exhaust. The system may comprises an exhaust conduit to conduct engine exhaust; a particulate filter disposed along the exhaust conduit; a reductant storage device coupled to the exhaust conduit and configured to store urea; and a controller comprising memory and a processor, the memory comprising instructions executable by the processor to supply the urea to the exhaust system upstream of the particulate filter when exhaust gas temperature exceeds a threshold value during particulate filter regeneration. In this way, urea stored on board for NOx reduction in an SCR, for example, can also be used for managing filter regeneration. Note also that urea injection may be used to manage other self-sustained and self-induced temperature increase conditions, such as sulfur regeneration, etc.